The disclosure relates to an actuator for high precision positioning and/or manipulation of components, in particular of optical elements or other functional elements in projection exposure systems for semiconductor lithography, and to a method for operating such an actuator. Here, the term “actuator” is to be understood as being synonymous with the terms “final controlling element” and “actuating element” that are likewise used.
There is a regular requirement for the abovenamed components to be positioned and/or manipulated in the nanometer range in order to be able to ensure the overall functionality of the higher level system. It is frequently necessary in this context to monitor the position of the positioned/manipulated components or the alignment thereof in space, with the aid of a high resolution and thus cost intensive and, as the case may be, susceptible measuring and control electronics.
The accuracy of positioning and/or manipulation of conventional systems is chiefly determined not by the actuator technology itself, but by the accuracy of the position measurement. In other words, the actuators can have smaller step widths than can be determined by the position measurement.
However, the step width of the drive of conventional actuators can change as a function of the load that acts on the movable part for the actuator. As a result of this, it can be impossible to calculate the output movement, and so the latter has to be monitored with a measuring system. In addition to this is the fact that slight deviation of the step width can build up over longer travel paths of the actuator.
The problems described are explained below with reference to piezo actuators described in German Published Patent Application DE 100 225 266 A1. DE 100 225 266 A1 describes an actuator for which the actuator movable part (i.e., the moving part of the actuator, which acts on the component that is to be manipulated and/or to be positioned) is driven forward via one or more advancing elements (“feet”) that are perpendicular to the movable part. Here, the advancing elements move in the direction of the movable part in a fashion perpendicular to their own longitudinal direction.
Since such a foot also exhibits a certain compliance in the direction of the effective direction of the actuator, the step width that is produced by the foot is a function, on the one hand, of the force that the foot itself can apply (advancing force), and on the other hand of the force against which the foot starts to work, or of the force that exerts tension or pression on the movable part of the actuator.
Consequently, a defined advancing force deflects the foot by a defined absolute value, but owing to the compliance of the foot there is superposed on this deflection a second deflection which results from the load on the movable part.
If, for example, a force acts on the movable part in the direction of advance, the step width becomes larger than the nominal step width, that is to say the step width to which the actuator is designed. If, by contrast, a force acts on the movable part against the effective direction, the step width becomes smaller than the nominal step width. In cases where the load on the movable part changes as the actuator travels, the step width can also change therewith. Consequently, the step width should be checked with an additional high precision displacement sensor but, for reasons of design space and manipulation, this is not always desired or possible.
Another type of high-resolution step drives is the inertial drive. With these drives, an advancing element (e.g., a piezoceramic) pushes the movable part slowly in one direction via a friction contact. In this process, the load on the movable part and the acceleration force on the movable part must be smaller than the transferable frictional force in friction contact. Subsequently, the advancing element is withdrawn with a jerk, the required acceleration force of the movable part being larger for the quick backward movement than the frictional force that can be transferred in the friction contact. The movable part therefore remains stationary, while the advancing element turns back in relation to the movable part. However, such drives have the disadvantage that they can exert only a slight force, since the force on the movable part together with the acceleration force (inertial force) of the movable part is not permitted to exceed the transferable frictional force in the case of the forward movement.
Since, in addition, the movable part cannot be secured when the advancing element is withdrawn with a jerk, the movable part can be “maladjusted” at this instant by an external force on the movable part.